ABSTRACT

Insulation design is one of the most important aspects of transformer engineering, particularly in high voltage transformers. Sound design practices, use of appropriate insulating materials, controlled manufacturing processes and good housekeeping ensure quality and reliability of transformers. Minimization of non-uniform fields and creepage stresses, improvement in oil processing and impregnation, elimination of voids, elimination of local high stresses due to winding connections/crossovers/transpositions, are some of the important aspects in the insulation design. In some critical cases, an electrostatic shield is used around the core/end limb facing the windings. The phase-to-phase insulation design is very similar but the stresses due to switching impulse and power frequency test levels need to be considered; most of the time the voltages between phases are 150% of phase-to-ground voltage levels. Due to the steady increase in transmission system voltages, the voltage ratings of power transformers have increased and hence insulation content in them contributes significantly to their cost.